I have resistivity data, ρ(B), at some fixed temperatures. To obtain the oscillatory part, ρosc, I have to subtract a background from ρ(B). For getting the background, one need to fit the ρ(B) data with a polynomial. What kind of polynomial (order of the polynomial) one should use? And what is the lower cut-off value of the magnetic field (B) from where I can fit the data?
Please see Phys. Rev. Lett 108, 216803 (2012) (doi:10.1103/PhysRevLett.108.216803), especially Fig. 2 and Fig. 3a. Authors do not specify what order of polynomial they have used, but it is easily seen that a straight line is not satisfactory. Thus the second degree polynomlal (a parabola) or maybe even third should do the job (you should use the smallest satisfactory degreee as the higher degree polynomials tend to oscillate). As to the second question: lower cut-off of magnetic field is may be roughly estimated as the one where your signal (after removing background) and the standard deviation of measured resistance are nearly equal to each other.
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